LimeSDR mini fails in QPSK modulation

Hello all,

Last few months i was completely working with GNU Radio and LimeSDR mini for a small project in which i wanted to send data using some QPSK Modulation. But this device is not able to modulate the baseband signal properly so that i am unable to receive any file at file sink block. Further i tried with GMSK and GFSK modulations, the device transmits it very efficiently. Also i used another SDR (HackRF One) with my same design and i was receiving a proper data at receiver end. So i concluded that it is problem with the Hardware which is not able to modulate it properly.

Can you please find me a solution for this so that i could use QPSK modulation in LimeSDR.

Any drivers have to be updated in GNU Radio??

Thanks & Regards,
Vinayak MK

Can you share a screenshot of your flowgraph or some constellation diagrams?

Can’t really help with only this part.

How does the constellation on TX look like?
Are you using only one LimeSDR mini or are you transmitting between two?
How does the constellation look on the RX?
Is AGC active on the RX?

I tried with one SDR and also with 2 separately.Tx constellation on GNU radio was looking good but spread-ed. At receiver also i am able to get some constellation ( i have used same plot for both Tx and Rx).How to know AGC is on at Rx? there is no such option available in LimeSuite Source (RX),then how can we make it active?

You are working on a very similar project like me. Your automatic gain control is not active because the LimeSuite block in gnuradio does not support it.

See this post

The problem is that the RX is not configured sensitive enough (which is why you need AGC) and that, if you use two boards, you need synchronization. This Wiki page @ the gnuradio wiki helped me a lot.

So it is possible to transmit with a LimeSDR, but youre probably gonna need to put some effort into it. I’ve been working on it for some time now, but then again, I am a complete noob in this sector.


If i need to use AGC, then how am i getting data when i uses the other modulation schemes such as GMSK or GFSK? I am pretty sure it is not the problem at the receiver side anymore. Because with HackRF the same design works perfectly.

Not sure why it works with GFSK. I tried to get GFSK running before starting to program blocks and do QPSK modulation, it didn’t work for me.

This is the constellation i am getting for GFSK

And at the file sink i received the text file also, using two mini boards and computers.

hello@maschi01 you can try the

screenshots block diagram in which you will get GFSK exactly by transferring the data.I have also attached the output constellation diagram.(Can use Limesdr-USB)for better output.
Soma Sundaram Ravindran

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@soma96be Thanks, I’ll try that out!

That is okay,so you have tried transferring the data(text transfer) using BPSK alone,or have tried with some 16QAM and 64QAM too…
Soma Sundaram Ravindran

I’m still working on it, but when it’s finished, I should be able to use BPSK and N-QAM to at least N=16. I don’t think SNR will allow any higher modulation though.
I will post a update in the above mentioned thread once I got something to show.

can you please send me the BPSK and QAM grc files …


Vinayak MK

@vinayak i will send you
Soma Sundaram Ravindran

hi this is bindu can u please send the same bpsk grc file to me. even im not receiving the data while transmitting the txt file

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could we transmit and receive a video file by using two limesdrmini? particularly by using GFSK can we transmit and receive a video file.(not live video, one mp4 file , which is around 2MB).